Primary productivity

Primary productivity
n., plural: primary productivities
[ˈpɹaɪməɹi ˌpɹoʊdʌkˈtɪvəti]
Definition: the production rate of biomass from inorganic molecules

Planet Earth is home to different types of life forms ranging from microscopic bacteria to giant whales and elephants. To keep the cycle of life running in a systematic fashion on the planet, the energy exchange must continue to happen via some processes or the other. As we all know that not all organisms can synthesize their own food (energy source) and hence rely upon other organisms for their nutritional requirements. In such a scenario, the production of the food sources from a non-assimilable form to an assimilable form becomes of significant importance. The process of the production of biomass via conversion of ‘non-assimilable’ inorganic molecules to ‘assimilable’ organic form of biomass for the survival and nutritional requirements of heterotrophic lifeforms is called primary productivity. The charge for this primary production is endowed upon the “autotrophs” of the ecosystem. Read on to learn what is the process of primary productivity, why is primary productivity important, the definition of primary productivity in ecology, primary productivity in environmental science, examples of primary producers, and much more.

Primary Productivity Definition

Before moving on to define primary productivity, let’s begin with the basic understanding of the topic “productivity process”.

Productivity in Ecology stands for the rate of biomass generation/energy addition/carbon fixation/organic matter added to an ecosystem. The standard units for expressing productivity are ‘mass per volume per unit of time’.

Now, productivity can be of different types namely primary and secondary productivity. Let’s introduce you to these two concepts and deliver an answer to the frequently asked question ‘what are primary and secondary productivity?’.

What is primary productivity?

Primary productivity in ecology refers to the production rate of biomass from inorganic molecules. Here, the main conversion is of inorganic minerals to organic biomass. This duty is performed by the autotrophs of the ecosystem, hence the productivity of autotrophs (primary producers) such as plants and some bacterial species is called the primary productivity.

Primary productivity is the rate while primary production is the amount. So, the definition of primary production in biology is a little different compared to primary productivity. Primary production is the amount of organic biomass produced in a given frame of time.

What is a primary producer?
A primary producer is an autotroph who’s capable of acquiring its energy from sunlight and elements of non-living origin or source. Examples: algae (thallophytes), higher plants (angiosperms), and some bacteria and protists. So, is grass a producer, the answer is yes!

What is secondary productivity?
Secondary productivity in ecology stands for the production of biomass from organic molecules. Here, the main conversion is of one form of organic molecules to some other form of organic molecules. This duty is performed by the “heterotrophs” of the ecosystem, hence the productivity of human beings, other animals, etc. is called the secondary productivity.

What are secondary producers?
Secondary producers are the herbivorous animals that feed on plants for their nutritional and energy requirements. As they consume primary producers (plants or algae) and themselves become the source of energy for their predators (tertiary producers), they are generally referred to as secondary producers or primary consumers; their predators are called tertiary producers or secondary consumers.

Some important pointers to primary productivity are:

• Performed by living organisms.
• The final product of primary productivity is a simple or complex “organic molecule”. (Example- carbon dioxide and water)
• Primary productivity forms the basis of all food chains and food webs of the ecosystem. Without the primary productivity at the base, no food chain can be established as the higher organisms are “incapable” of utilizing the inorganic molecules as such (hence called non-assimilable inorganic compounds). This implies the importance of primary productivity.

Types of Primary Productivity

Now we know what productivity or production means in biology. In addition to that, we also understood the 2 types of productivity. Now, since we understand and know how to define primary productivity, let’s take a quick overview of the two types of primary productivity.

1. Gross primary productivity (ecology) refers to all the carbon that’s fixed by the primary producers of the ecosystem i.e., the main product of photosynthesis.
2. Net primary productivity (ecology) refers to the difference between all the carbon that’s fixed by the primary producers of the ecosystem and the part of that fixed carbon that the primary producer utilizes for its own cellular respiration needs.

   

Gross Primary Production and Net Primary Production

To understand the concept of gross productivity and net primary productivity in detail, look at the table below.

Data source: Akanksha Saxena of Biology Online

Formula for productivity calculation

For the productivity calculations and productivity equation, there’s a well-established formula, the GPP formula.

NPP = GPP – RR

where,
NPP= Net primary productivity
GPP= Gross primary productivity
RR= Respiratory Rate

This formula is important as it diligently associates GPP to NPP and also makes the metabolic losses and respiratory losses of autotrophs/producers count.

Generally, NPP is calculated and measured at the level/scale of “ecosystem” and over relatively longer frames/intervals of time (such as a year/s).

You should be able to answer all the basic questions now about what is gross primary productivity and net productivity (GPP and NPP)!

Routes of primary production

Primary production can happen via both the processes called photosynthesis and chemosynthesis.
When primary productivity happens using light as the sole source of energy, it’s called photosynthesis. Such organisms are photo-litho-trophic by their nature.

Examples: Photosynthetic algae, plants, photolithotrophic bacteria like purple bacteria (example- Chromatiaceae), green bacteria (example- Chlorobiaceae), and Cyanobacteria.

When primary productivity happens using “energy from redox reactions (oxidation or reduction reactions of inorganic chemical compounds)” as the sole source of energy, it’s called chemosynthesis. Such organisms are chemo-litho-trophic by their nature

Examples: Chemolithotrophic bacteria like iron bacteria (example- Acidithiobacillus ferrooxidans), methanogens (example- archaea), aerobic hydrogen bacteria (example- Cupriavidus metallidurans), etc.

Factors limiting Primary Productivity

There are multiple factors that limit the primary productivity to reach the theoretically highest possible optimum. Let’s try to list them.

• Limited part of sunlight reaches the primary producers: Till the sunlight reaches the autotrophs, a large amount of it has been filtered off. It’s not that the full amount of sun’s radiation reaches the Earth’s surface, a large amount (almost half) is reflected back by various atmospheric layers of the Earth.

• Limited part of sunlight can be utilized by the primary producers: When we look at the range of light finally reaching primary producers at last after various filtering processes, it’s only about half of the wavelengths that plants can utilize. Plants can only utilize ~400-700 nm wavelength spectrum of sunlight. As we know, this spectrum is called the PAR (Photosynthetically Active Radiation).

• Location: Different locations of Earth receive different amounts of sunlight, hence the autotrophs in different locations have different productivity. (More in the equatorial region and less in the poles).
• The efficiency of photosynthesis: The process of photosynthesis isn’t 100% efficient.
• Availability of sunlight
• Availability of nutrients and minerals
• Availability of water

NOTE: Primary productivity is not limited by time.

Terrestrial Production

When we talk about the primary production and primary productivity on the land, i.e., terrestrial primary production, we particularly emphasize vascular plants. Only a very little fraction of terrestrial primary production is credited to algae and nonvascular plants (bryophytes like mosses and liverworts).

Since we all know that vascular plants have evolved much later than lower plants, it’s evident that terrestrial primary production began after the aquatic primary production on the face of Earth.

Terrestrial primary production is dependent on many factors like local hydrology and temperature. A very important factor that determines the amount of terrestrial primary production is PAR (photosynthetically active radiation).

PAR is the main source of direct energy for photosynthetic plants.

Terrestrial primary production reduces in its ‘amount’ and ‘extent’ when the determining factors push towards the extreme limits like very extreme temperatures (both hot and cold), lack of water resources, etc.

Terrestrial primary production is also ensured by the primary producers by adaptive mechanisms to compensate for the extreme climatic conditions. Example: CAM and C4 plants.

Terrestrial primary production varies in the same area depending upon the different seasons.

Where is primary productivity highest? In the context of terrestrial ecosystems, the primary production is highest in the tropical rainforests.

Which type of biome is the most productive? The answer is again tropical rainforests.

Oceanic production

When we talk about the primary production and primary productivity in the ocean, i.e., aquatic primary production, we particularly emphasize algae.

This is exactly opposite to terrestrial primary production as here only a little fraction of the primary production is contributed by vascular plants.

There is a variety of eukaryotes that contribute to the primary productivity of the ocean. Examples are green algae (Chlorophyta), brown algae (Phaeophyta), red algae (Rhodophyta), and also a diverse group of unicellular groups. Some examples of angiosperms (vascular plants) in the ocean are seagrasses.

A major part of primary production in the ocean is taken care of by free-living microscopic organisms called ‘phytoplanktons’- producers in the ocean.

Algae increase the dissolved oxygen and take care of aquatic primary productivity.

Aquatic primary production is dependent on many factors like salinity of water, availability of light in the different zones of the ocean, the source of energy for photosynthetic organisms, and availability of mineral nutrients, etc.

The limiting factors for aquatic primary production are very different from those for terrestrial primary production. Let’s discuss some important ones here.

Light

Light is one of the most important factors that determine oceanic production and aquatic productivity.
The part of the ocean that receives sunlight= Photic zone/Euphotic zone. It’s quite thin and only on the surface (some 100m maximum). It’s in this photic zone that the “maximum primary production” by phytoplanktons takes place. Wind flow and speed can lead to substantial mixing of the upper photic zone of the ocean. This significantly affects the primary productivity of phytoplankton producers residing in the upper photic zones of the ocean.

Nutrients

Nutrients are another important factor that determines oceanic production and aquatic productivity.
Under the gravitational effect, nutrients in the ocean tend to sink down from the upper photic zones to the deeper ocean zones and eventually to the ocean floor. In such a scenario, the mixing of the ocean water via wind flow becomes primarily important. This wind flow ensures that the nutrients which are essential for the primary production by the phytoplankton in the photic zone are continuously replenished.

Iron

Iron is another significantly important factor that determines oceanic production and aquatic productivity.
Although a micronutrient, iron plays an essential role in primary productivity. It’s used as an irreplaceable cofactor in primary production enzymes and processes like nitrate reduction and nitrogen fixation.
Major inputs of iron for primary production come from: “dust from the Earth’s deserts”.

Measurement

When calculating primary productivity, it can be taken in different methods.

• Gross primary productivity vs Net primary productivity
• Terrestrial primary productivity or Aquatic primary productivity

Some general observations for these measurements and related challenges are:

• Gross primary productivity: usually harder than net primary productivity to calculate.
  Reason: GPP takes into account respiratory losses and maintenance losses.
• Terrestrial primary productivity: usually harder than aquatic primary productivity to calculate.
  Reason: Terrestrial primary productivity is accounted for by both shoot and roots of plants and trees.
• And since a great proportion of flora is below the ground (root organs and tissues), this poses challenges for the primary productivity measurements.

Terrestrial

We will briefly look into the methodologies, challenges, and importance of primary productivity measurements in the terrestrial ecosystems here.

• Scientists usually do the measurements for NPP and not GPP.
• Even when they do NPP measurement, they are challenged by multiple factors like:
  • Incapability to measure “below ground productivity” (BIGGEST challenge)
  • Incapability to measure herbivory
  • Incapability to account for turnover and litterfall
  • Incapability to account for volatile organic compounds and root exudates
  • Incapability to account for symbiotic microorganisms
• All these factors lead to the underestimation of NPP.
• There’s a well-defined terminology for the loss of NPP via below-ground productivity loss via roots. It’s called “BNPP: below-ground NPP”.
• BNPP measurement and calculation: Scientists usually measure BNPP as a “ratio of ANPP (above-ground NPP): BNPP (below-ground NPP)”. They tend to usually avoid direct measurements.

Grasslands

We will briefly look into the methodologies, challenges, and importance of primary productivity measurements in the grassland’s biome here.

• It’s recommended to make both the measurements: “standing live biomass” and “dead biomass” in grasslands.
• This repeated measurement gives highly accurate estimates for grasslands.
• For grasslands with very large turnovers, rich and high interspecific variation in addition to rapid decomposition rates, scientists need to particularly do these repeated measurements.

Forests

We will briefly look into the methodologies, challenges, and importance of primary productivity measurements in the forest’s biome here.

• As compared to grasslands, the measurement tools and methodologies used for forests are wide-ranging and more diverse.
• For ANPP (above ground NPP), stand specific allometry and litterfall are regularly used to estimate the forest primary productivity.

Aquatic

We will briefly look into the methodologies, challenges, and importance of primary productivity measurements in the aquatic ecosystems here. In aquatic ecosystems, primary productivity measurements are usually based on 6 main types of techniques that are listed below:

• Variations in O₂ (oxygen) concentration within a sealed bottle.
• Incorporation of inorganic carbon-14 into organic matter.
• Stable isotopes of O₂ (oxygen).
• Stable isotopes of Carbon (12C and 13C)
• Oxygen/Argon Ratios
• Fluorescence kinetics

Global

Measurements of primary productivity (PP) are important because:

• PP is an indispensable part of the global carbon cycle. Hence, measurements of primary productivity become pivotal in order to understanding Earth science.
• Measurements of primary productivity are difficult to conduct at a global scale since a humongous number of factors affect it like weather, availability of sunlight, water, nutrients, etc.
• When looking at the global measurements of primary productivity, it’s important to note that PP varies enormously between land and the oceans.

Estimates

For the measurements of primary productivity (PP), there exists a range of proxies. One of them is barium. The concentration of barium in oceanic sediments escalates and increases in accordance with the primary productivity (PP) at the surface.

Human Impact and Appropriation

Humans do play a part in Earth’s primary productivity. But the kind of influence they have on it is disproportionate. In the sphere of ecological economics and sustainability, a new term has been coined called “HANPP”. HANPP stands for Human Appropriation of Net Primary Production. The concept was introduced by Josep Garí in 1996. HANPP estimates the anthropogenic appropriation of NPP. HANPP is applicable to different geographical zones, and scales. It can also be applied on a global scale.

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References

• Florencia Montagnini, Carl F. Jordan. (2005). Tropical forest ecology: the basis for conservation and management.
• Irena Šímová, David Storch. (2016). The enigma of terrestrial primary productivity: measurements, models, scales and the diversity–productivity relationship. Ecography. https://doi.org/10.1111/ecog.02482
• Zhao, M., & Running, S. W. (2010). Drought-induced reduction in global terrestrial net primary production from 2000 through 2009. Science (New York, N.Y.), 329(5994), 940–943. https://doi.org/10.1126/science.1192666

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